In the U.S. Pat. No. 3,895,054 there is disclosed a process for the manufacture of urethanes (carbamic acid esters) by reacting an organic compound containing at least one hydroxyl group, e.g., ethyl alcohol, with carbon monoxide and a nitrogenous organic compound at elevated temperature and pressure in the presence of a selenium catalyst and a base and/or water.
The present invention is directed to a simple and effective method of recovering the selenium from urethane solutions produced, for example, by the above described process and containing selenium which may be in the form of selenium per se, inorganic selenium compounds and organoselenium compounds. Because of the high reactivity of selenium, when used as a catalyst as in the above reaction, its combination with organic compounds very frequently results in the formation of organoselenium compounds, such as benzoselenazol and diethyl diselenide, which remain in solution with the urethane reaction product. In such reaction a portion of the selenium catalyst such as selenium per se or selenium dioxide, selenium disulfide, selenium oxychloride, etc. is converted to one or more organoselenium compounds which may be classified generally as selenols, selenides, benzoselenazoles, esters of selenocarbonic acid, selenic acid and selenious acid, selenones and the like. The type and number of organo-selenium compounds which may be formed in a function of the reaction conditions used to produce the urethanes such as time, temperature, pressure and solvent. In addition, the inorganic selenium compounds used as catalysts or formed in the reaction may also remain in solution with the urethane product.
Because of the cost of selenium, it is essential that as much of the selenium be recovered from the urethane reaction product as is possible and from the inorganic or organoselenium compounds in a form suitable for reuse as a catalyst.
Prior art processes have been proposed for the recover of selenium from certain organic reaction products including urethane solutions. However, such prior art processes which disclose extraction processes as well as adsorption and chemical complexing of the selenium compounds still leave appreciable amounts of the selenium-containing compounds in the urethane solution.
U.S. Pat. No. 3,084,994 discloses a method for the recovery of selenium from a gas containing selenium together with aldehydes and nitriles using water sprays to collect the selenium and atomizing the selenium-containing water in molecular oxygen-containing gas in contact with an oxide of copper, iron or nickel at 500.degree. C. to convert the selenium in the water solution to selenium dioxide.
U.S. Pat. No. 1,915,703 describes a method for the recovery of spent selenium from hydrocarbon conversion products such as naphtha obtained in the pyrolysis of the hydrocarbons in the presence of selenium, by extracting the selenium with an alkaline solution and subjecting the alkaline solution to treat with an oxidizing agent such as hydrogen peroxide, oxygen and air to precipitate selenium which may be filtered from solution.
A number of recently issued patents, namely, U.S. Pat. Nos. 4,055,630; 4,055,629; 4,041,139; 4,038,377; 4,038,376; 4,038,375 disclose the removal of selenium compounds from urethane solutions using copper (I) chloride and an amine or nitrile complexing agent, mixtures of metals and metal oxides of Group IB, IIB, VIB, and VIIIB elements, liquid-liquid extraction with water and a saturated aliphatic or acyclic hydrocarbon, liquid-liquid extraction of a hydrocarbon solvent solution of selenium-containing urethanes with aqueous ammonium hydroxide or alkali metal hydroxides, metal oxides and an aqueous mercuric salt compound respectively.